Directional antenna



H. KRUTTER May 20, 1952 DIRECTIONAL ANTENNA Filed March 8, 1945 FIG. 2

INVENTOR. HARRY KRUTTER WMQ/M/ ATTORNEY Patented May 20, 1952 UNITED STATES T OFFICE 2,597,339 DIRECTIONAL ANTENNA Application March 8, 1945, Serial No. 581,627

9 Claims. 1

This invention relates to antennas for radio objest-locating systems and particularly to antennas having a grating type or grate-like parabolic reflector adapted to radiate electromagnetic energy in alternately different energy distribution patterns.

In certain object-locating systems such as used for tracking and navigation it is desirable to radiate energy in a distribution pattern having a relatively wide angle in one plane or dimension, such as a fan-type beam, for searching for a target, and in another distribution pattern narrow both in azimuth and elevation, such as a pencil beam, to track on or navigate toward the target. The present invention provides an antenna system including a reflector which is adapted alternately to radiate a beam of energy which scans a field in space with either a fan-type beam or pencil beam in a conical or like manner.

It has been found that a desirable fan-type beam is one in which the distribution of energy in one plane throughout a relatively wide angle follows a csc e variation (or be such that the variation of energy density versus the radiation angle is a cosecant-squared function), being the angle of radiation measured from the normal axis of the radiatin means. Such a distribution pattern provides relatively constant echo strength for targets located on the earth or at equal altitudes in space, regardless of whether the target is close in or distant, and it eliminates the need of tilting the antenna to obtain adequate coverage of the area under observation. Usually a cosecant-squared beam is obtained at a loss of some of the antenna gain because of diffusing the radiating power over a, large area. Hence, it is sometimes preferable to use a pencil beam for general searching and after the target is found to switch to a cosecant-squared beam, as less antenna gain is needed, to track on or navigate towards the target.

While a smooth, continuous metallic surface is the simplest and usually the best form of microwave reflector from the standpoint of eificient reflection, there are several applications for which some sacrifice of reflector efficiency is justifled and for which a partially open reflector in the form of a, grating, screen or perforated sheet will have advantages, such advantages including:

(1) Reduced wind resistance.

(2) Reduced weight/strength factor.

(3) Increased ease of fabrication and assembly.

('4) Visual transparency, and

(5) Adjustability of reflector shape.

Accordingly this invention contemplates as antenna system having a partially open reflector, particularly a grating type reflector, which is adapted to provide alternately different types of energy distribution patterns as referred to herein.

One of the objects of the present invention is to provide an antenna having a partially open refiector adapted to radiate a beam of energy which scans a field in space with either a fan-type beam or a pencil beam,

Another object of the invention is to afford complete radiation coverage throughout a relatively wide angle in elevation (that is, in one plane) and in conformity with the desired mathematical relation between the intensity of radiated energy and angle of elevation.

Still another object of the invention is to provide an antennahaving a partially open reflector adapted to radiate a beam of energy in a distribution'pattern which is narrow both in azimuth and elevation and alternately radiates a beam in a distribution pattern of csc a configuration.

A further object of the invention is to provide an antenna in which at least a portion of the refleeting surface of a parabolic grating type reflector is adapted to be quickly and conveniently adjusted, or supplemented by an auxiliary grating surface, to provide a modified reflecting surface and thus alternately different types of energy distribution patterns. Other novel features and advantages will be apparent from the following disclosure of the invention.

In the drawings:

Fig. 1 is a diagrammatic side view of a parabolic reflector illustrating one embodiment of the invention;

Fig. 2 is a diagrammatic side view showing the radiation patterns produced respectively by the parabolic reflector of Fig. 1 when used in the conventional manner and in the modified form according to the present invention;

Fig. 3 is a sectional side view of the upper half of the parabolic reflector similar to that shown in Fig. 1 illustrating structural features of one embodiment of the invention;

Fig. 4 is a partial top plan view of the reflector shown in Fig. 3;

Fig. 5 is a partially broken away front view of the upper half of the reflector shown in Figs. 3 and 4; and

Fig. 6 is a sectional side view of the upper half of a parabolic reflector showing a modification according to the present invention.

Referring to Fig. 1 of the drawings, a radiating element or source I0, such as a dipole and parasitic element, fed by a wave guide or coaxial line II, is constructed and located in a conventional manner for illuminating a reflector I2, Reflector I2 is formed as a substantially parabolic surface and preferably a paraboloidal surface having its focal point at the apparent center of radiation of the radiation source or element II).

In conventional antenna systems embodying a reflector of the above kind it i well-known to form the reflecting surface of a thin conducting sheet of metal or other suitable material. However, according to the present invention, reflector I2 is formed as a partially open type reflector, at least a portion of the reflecting surface of the reflector preferably being in the form of a grating. By way of example, and to facilitate illustration, an upper half portion of the reflector I2 as shown in the drawings has a portion I3 of the reflecting surface thereof formed as a grating. It will be understood, however, that the invention contemplates a reflector made entirely as a grating or having only certain portions thereof formed as a grating, depending on the desired use to which the reflector is to be applied.

Grating portion I3 comprises a plurality of curved, uniformly spaced parallel bars I 4, the forward surfaces of bar I4, nearest the radiating element It, defining a portion of a parabolic reflecting surface I of the reflector I2. Bars I 4 may be round bars or preferably thin strips of metal or other suitable electrically conductive material, preferably arranged with their planes perpendicular to the plane of the reflecting surface I5 and with their longitudinal front edges being disposed substantially flush with or slightly in front of the surface I5. It is desirable that the percent of open area of the grating portion I3 be large relative to the areas of the edges of the bars or strips I4. It is also desirable that the distance between adjacent bars or strips be not more than half the wavelength of the radiant energy so that the surface formed by the front edges of the bars or strips I4 will effectively act as a part of the continuous parabolic reflecting surface I5 of the reflector I2 whereby surface I5 is adapted to direct a pencil beam of radiant energy as indicated by the solid line configuration of Fig. 2.

Referring to Fig. 1, another grating section I6 is shown disposed behind the grating I3. Grating I6 comprises a plurality of bars or strips I1 constructed and arranged in a manner similar to that described above with reference to bars or strips 14. Grating section I6 is adapted to be moved from its normally ineffective position at the rear of reflector I2 to the effective position in front of that portion of reflector surface I5 defined by grating portion I3' as indicated in broken or dotted lines (Fig. 1).

The reflecting surface I8 defined by the front edges of bars or strips I! may be of any desired shape and size. Preferably surface I8 is disposed at an angular relation to surface I5 or to a horizontal plane through the normal axis of reflector I2 and is so shaped that when in position in front of surface I5 it is capable of deflecting radiant energy from the radiating element I0 to produce a fan-type beam having distribution pattern substantially as indicated by broken lines in Fig. 2.

It will be apparent that while the above description refers to gratings I3 and I6, in association with one half or, as shown, the upper half portion of reflector I2, it will be understood that the other half or lower half section of reflector I2 may also be provided with gratings similar to gratings I3 and I5. Thus, grating section as I6 may be adapted to be positioned in front of either or both of gratings I3. Therefore, it will be apparent that the reflecting surface of reflector I2 is adapted to be modified by the positioning of one or more grating sections I6 to obtain any desired energy distribution pattern. It will also be, appreciated that scanning with the desired beam pattern may be accomplished by rotation of the antenna about a vertical axis in a wellknown manner.

Referring now more particularly to Figs. 3, 4 and 5, structural means is shown for mounting and moving the grating sections I5 according to one embodiment of the invention. Reflector I2, with grating portion I3 and a grating section It, is constructed and arranged in the manner described with reference to Fig. 1. Bar or strips I! are disposed respectively in the horizontal planes between the bars or strips I4 and are supported by arms I9. Arms I9 may be secured in any suitable manner to bars II, preferably at the extremities thereof and, if desired, at intermediate points along the bars IT. Also, arms I9 may be integral with the bars or strips I'l. Arms I9 preferably are disposed so that they extend substantially parallel with the normal axis of reflector I2 and are carried by or mounted on upstanding support members 20 in any suitable manner. For example, arms I9 may be rigidly secured to supports 20 or slidably mounted thereon as more fully described hereinafter.

Supports 20 may be bars or beams of any suitable rigid material and of any desired shape. As shown more clearly in Figs. 4 and 5, there are, for example, three supports: 20b, 20c and 20d. Supports 20b and 20d carry the arms I9 supporting the extremities of bars I1 and 290 is an intermediate support here shown located at the approximate center position between supports 20b and 20d. The lower end of supports 20, or the ends nearer the axis of reflector I2, may be rigidly connected to or carried by a rod or shaft 2I mounted in suitable bearings or preferably pivotally or slidably mounted in brackets 22 as more fully described hereinafter. The opposite ends of supports 20 may be connected by a bar or rod 23 to improve rigidity and, if desired, to provide an additional means of support.

Brackets 22 are preferably located at the ends of, and if desired at intermediate points along, shaft 2| and may be suitably secured to the reflector I2 or its mounting structure (not shown). As illustrated in Fig. 3, each bracket 22 is provided with a slot 24 having its longitudinal axis substantially parallel with the axis of reflect'or I2. Shaft 2I- is mounted in and adapted to slide in slots 24 from one end of the slot to the other. By making slots 24 of correct length, grating I6 supported by shaft 2| is adapted to be moved from the normally ineffective position at the rear ofreflector I2 as indicated by dotted lines in Fig. 3, to pass between the grating bars or strips I4, and into the effective position in front of reflecting surface I5 as indicated by solid lines in Fig-.3 and vice versa. Any suitable electrical or mechanical means such as the lever mechanism generally designated 25 may be used for selectively moving the grating I6. Slot 24 may also be L-shaped with its short leg at the forward end extending in a downward direction or toward the axis of reflector [2 so that the shaft 2|, after moving forwardly along the long leg of slot 24, may move downwardly, thereby moving the whole grating structure 16 forwardly and then downwardly so that bars I! may more effectively screen or shield bars or strips M. This feature will prove particularly desirable when bars or strips I! are mounted in such a way that they depend slightly below the forward ends of 'arms [9.

The correct or desired shape of reflecting surface l8, Fig. 1, formed by the forward edges of bars l'l may be determined by making the arms l9, Figs. 3 and 4, of the correct length. Thus, to obtain the desired angular relation between the surfaces and I8, as indicated by a in Fig. 1, the uppermost arms l9 or the arms farthest away from the reflector axis will be made the longest, and each succeeding lower arm l9 or arm nearer the reflector axis will be made progressively shorter.

Fig. 6 illustrates a modification in which the arms l9a, corresponding to the arms I9 in the previously described embodiment, are provided with slots 21 having their longitudinal axes along the lengths of arms lSa. In this modification arms I9a may be supported or carried on supports 2011 by lugs, nuts or rivets 28 through any suitable apertures in supports a and through slots 21. Slots 2'! may be made of various lengths or, if desired, provided with spaced notches or calibrations whereby arms I90, may be moved individually to any predetermined position relative to supports 20a to obtain the desired shape of the reflector surface as [8, Fig. 1. Thus, it will be apparent that by proper adjustment of the effective and relative lengths of arms |9a any desired shape and angle of reflecting surface as I8 may be obtained. Hence, any desired form of radiation pattern including a csc 0 pattern may be produced quickly and conveniently by moving a grating as [6, Fig. 1, into position in front of reflector l2 according to this invention.

Fig. 6 also illustrates a further modification. Instead of providing a separate auxiliary grating section as 16, a grating portion l3a may itself be supported on the arms 19a and mounted thereon in the manner described above with reference to Figs. 3, 4 and 5. Thus the grating section I30. may be moved forward as a unit into the dotted-line position of Fig. 6. Also, if desired, the reflecting surface formed by the forward edge of bars Ma may be properly shaped by moving arms |9a relative to supports 20a as hereinabove described.

Additional brackets 29 with suitable slots 30 may be provided for receiving and supporting the upper connecting bar or rod 23 or 23a. Slot 30 may, if desired, be made longer than slots 24a, thus enabling the grating [3a to assume an angle in the forward position relative to the angle of its normal position relative to the axis of reflector [2. Also, supports 2011 may be rotatably or pivotally mounted in brackets 22a 'whereby the movable gratings as H5 or I3a may be moved forward and tilted.

From the above description it will be evident that in accordance with the present invention there is provided an antenna structure comprising a parabolic reflector and a focally located feed element adapted to provide a pencil type directive radiation pattern. The parabolic reflector has an asymmetrically located portion of 6 its surface comprised of a number of parallel and spaced reflecting strips. In one embodiment of the invention there is provided a second number of parallel reflecting strips which are proportioned and arranged for movement from in back of the reflector through the spacings of the spaced strips and to a position where, by reflection, they modify the pattern to a fan type pattern of asymmetrical shape corresponding substantially to a cosecant squared energy distribution. -In another embodiment of the invention the spaced reflecting strips of the parabolic reflector are provided with means for being moved toward the feed element to a reflecting position to modify the pattern to a fan type pattern.

While preferred embodiments of the present invention have been illustrated and described and modifications thereof have been referred to in the present description, it will be understood that the invention is capable of further modification and improvement without departing from the spirit of the invention. For example any other suitable means of supporting and moving the grate-like portionsof the reflecting surface than the means specifically described herein may be used. Hence, it is not desired that the scope of the invention be limited to the precise details set forth. 7

Having thus described the invention, what I claim as new and desire to secure by Letters Patent is:

1. An antenna for use in radio object-locating systems comprising radiating means for emitting and receiving high-frequency energ a reflector having a normally parabolic reflecting front surface for reflecting said energy in a pencil-type beam, at least a portion of said reflecting surface being afforded by a plurality of curved parallel strips of conducting material forming a gratelike reflecting surface, a group of parallel curved strips adapted to interleave with said plurality of strips and forming a second grate-like reflecting surface suitably mounted in normally ineffective position at the rear of said first-mentioned grate-like surface and adapted to be moved through the spaces of and into effective position in front of said first-mentioned gratelike surface, said second surface when in said effective position reflecting a portion of the energy asymmetrically to cause the total energy to be reflected in a fan-type beam, and means for moving said second grate-like reflecting surface between said ineffective position and said effective position.

' 2. An antenna as claimed in claim 1 including a supporting structure for the group of strips, said structure having at least one support member movably mounted relative to thereflector, a plurality of arms corresponding to the number of strips of said group mounted on said support member and extending angularly therefrom, said strips of said group being respectively secured to said arms, movement of said support member toward said reflector being effective to move said group from the ineffective position thereof to the effective position thereof, during which movement said strips of said group and said arms are adapted to pass between the strips of the reflector, the relative lengths of said arms determining the desired shape of the reflecting surface formed by said strips of said group.

3. An antenna as claimed in claim 1 wherein means are provided for moving the second surface to an effective position asymmetric to the direction of the pencil type beam to reflect a part of the radiant energy asymmetrically to provide a radiation pattern which is substantially apower function of the cosecant of the radiation angle.

4. An antenna for use in radio object-locating systems comprising radiating means for emitting and receiving high-frequency energy, a reflector having a normally parabolic reflecting surface for reflecting said energy in a pencil-type beam, at least a portion of said parabolic reflecting surface comprising a plurality of parallel strips of conducting material forming a grate-like reflecting surface, movable means mounted adjacent said reflector for supporting said grate-like portion independently of the remaining portion of said reflector, means for moving said supporting means to move said grate-like portion relative to the remaining portion of said reflector to a position between said radiating means and said parabolic reflecting surface to transform said pencil-type beam into a distribution pattern having a relatively wide angle in one plane.

5. An antenna structure comprising a parabolic reflector-and a focally located feed element adapted to provide a pencil type directive pattern, said parabolic reflector having an asymmetrically located portion of its surface comprised of a number of parallel spaced reflecting strips, a second number of parallel reflecting strips proportioned and arranged for movement 9 from in back of said reflector through the spacings of said spaced strips and toward said feed element to a reflecting position which modifies said pattern to a fan type pattern of asymmetrical shape corresponding substantially to a cosecant squared energy distribution.

6. A directional antenna structure-comprising a radiating element, a reflector having a first reflecting surface partially surrounding said radiating element and fixed relative to said radiating element, said first reflecting surface having a portion thereof cut away, said reflector having a second reflecting surface movable toward and away from said radiating element, and means located behind said first reflecting surface and extendable through said cutaway portion for moving said second reflecting surface, whereby the sharpness of the radiation pattern of said directional antenna structurevaries in accordance with the position of said second reflecting surface relative to said radiating element.

7. A directional antenna structure according to claim 6, wherein said second reflecting surface comprises a plurality of bars forming a grating.

8. A directional antenna structure according to claim 7, wherein said first reflecting surface has a parabolic shape, said radiating element being at the focus of said first reflecting surface, said cutaway portion being asymmetrically disposed in said first. reflecting surface, said grating being movable to at least two positions, one of which is located Within said cutaway portion where said grating is adapted to form a continuation of the parabolic shape of said first reflecting surface, whereby said antenna structure radiates a pencil-type beam, and the other of which is located obliquely relative to the axis of said first reflecting surface between said cutaway portion and said radiating element, whereby said antenna structure radiates a cosecant-squaredtype beam.

9. A directional antenna structure according to claim 6, wherein said first reflecting surface has a parabolic shape, said radiating element being at the focus of said first reflecting surface, said cutaway portion being asymmetrically disposed in said first reflecting surface, said reflector having a third reflecting surface including a first plurality of bars forming a first grating, said first grating being fixedly disposed within said cutaway portion and adapted to form a continuation of the parabolic shape of said first reflecting surface, said second reflecting surface including a second plurality of bars forming a second grating, said second grating being movable to at least two positions, one of which is located behind said first grating, and the other of which is located. obliquely relative to the axis of said first reflecting surface between said first grating and said radiating element, said second grating passing through the spaces in said first grating in being moved from said one position to the other, whereby said second rating is ineffective in said one position thereof and said antenna structure radiates a pencil-type beam, and said second grating is effective in said other position thereof for modifying the directivity of said antenna structure so as to radiate a cosecantsquared-type beam.

HARRY I UUTTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,906,546 Darbord Mar. 2, 1933 2,078,302 Wolff Apr. 27, 1937 2,452,349 Becker Oct. 26, 1948 2,489,865 Cutler Nov. 29, 1949 FOREIGN PATENTS Number Country Date 694,523 Germany July 4, 1940 

